1. Field of the Invention
The present invention relates to a stacked, flow through capacitor and more particularly to a stacked, flow through, electric, double-layer capacitor. The invention further relates to a method of treating liquids containing ionic substances with a stacked, flow through capacitor.
2. Description of the Related Art
Flow through capacitors (flow through, electric, double-layer capacitors) utilize working electrodes of high surface area materials to electrostatically absorb, electrochemically react, or catalytically decompose substances that need to be removed or altered in fluids such as gases and liquids (aqueous and non-aqueous solutions). Referring to the technology by which ionic substances are electrostatically removed from water phases containing them by means of such flow through capacitors, the following published literature is available.
(1) U.S. Pat. No. 5,192,432 and U.S. Pat. No. 5,196,115 each discloses a flow through capacitor for coulostatic (constant-charge) chromatographic columns for use in the purification of liquids, comprising a plurality of adjoining layers such as a first conductive backing layer, a first high-surface-area conductive layer, a first nonconductive porous spacer, a second conductive backing layer, a second high-surface-area conductive layer, a second nonconductive porous spacer, etc., as wound together in a spiral configuration. It is also mentioned in the specifications of those patents that the above capacitor can be used in the purification of water containing ionic substances such as sodium chloride.
(2) Japanese Kokai Tokkyo Koho H5-258992 (corresponding to U.S. Pat. No. 5,415,768, U.S. Pat. No. 5620597, and U.S. Pat. No. 5,748,437) discloses not only a wound, flow through capacitor but also a stacked washer, flow through capacitor.
(3) Japanese Kokai Tokkyo Koho H6-325983 (corresponding to U.S. Pat. No. 5,538,611) discloses a flat plate, flow through, electric, double-layer capacitor comprising active carbon layers made predominantly of high specific surface area active carbon with an electrically insulating porous flow-through sheet separator interposed therebetween, a collector disposed externally of each active carbon layer, and a retaining sheet disposed further externally thereof. Generally, a frame gasket is interposed between the collector and the retaining sheet. To treat a liquid with this flow through capacitor, application of a DC constant voltage to the collectors and short-circuiting or reverse connection between the collectors is performed in an alternating manner while an ionic substance-containing liquid is passed through the capacitor.
The spirally wound, flow through capacitor (1) mentioned above involves assembling difficulties. For instance, it is difficult to prevent channeling on introduction of a liquid and to effect the tightening required in winding with good workmanship. Channeling occurs along the central support, peripheral edge, and/or front and back seals (depending on whether the direction of flow of the liquid is perpendicular or parallel to the capacitor axis). This problem must be overcome with a silicone cement or other adhesive and the literature mentions an epoxy resin as an end sealant. Thus, having a structure such that a plurality of adjacent layers is spirally wound, the wound, flow through capacitor tends to present the channeling problem and when the capacitor is applied to the purification of liquids containing ionic substances, the rate of elimination of the ionic substances fluctuates unsteadily during the purification process and, moreover, the elimination rate tends to be low on the average. Furthermore, in such a wound structure, the correct positioning of leads from the collectors is difficult. Therefore, considerable difficulties must be overcome when a wound, flow through capacitor of this type is applied to the purification of liquids on a commercial scale.
The stacked, washer electrode, flow through capacitor mentioned under (2) comprises a stack of disk-shaped electrodes and, as such, is disadvantageous costwise and practically ineffective because, of the constituent material having a given surface area, the effective area is small, the number of parts required is large, and use of a central support is essential.
Referring to the flat, plate, flow through capacitor (3), frame gaskets are generally used but because the interlayer contact is not uniform, the channeling tends to occur upon introduction of a liquid. Moreover, the volume of the liquid that can be treated with one flow through capacitor is limited so that there is room for improvement for commercial application.
Under the circumstances, the present invention has for its object to provide a flow through capacitor which features a high effective utilization rate of component parts, is easy to assemble, does not essentially require sealing around electrodes, is free from the channeling problem, and is conducive to a marked increase in the treatable load per unit time while upholding the rate of elimination of ionic substances, thus being very suited for commercial application. It is a further object of the invention to provide a method of treating liquids using said flow through capacitor.
The flow through capacitor of the present invention is a capacitor consisting essentially of a separator 1 and, as disposed on either side thereof, an electrode 2 and a collector 3 in that order.
This flow through capacitor is characterized in that each of the separator 1, electrode 2 and collector 3 is made of an independent or folded flat sheet having a polygonal configuration,
that the separator 1, electrode 2, and collector 3 are stacked in the order of
[{fraction (3/2)}/xc2xd]n/3
xe2x80x83where n represents an integer of not less than 20,
that the separator 1, electrode 2, and collector 3 are respectively comprised of a polygonal sheet having one or a plurality through-holes h for passage of a liquid, with the through-hole or holes h in each sheet being aligned with the corresponding through-holes in the other sheets to form an internal flow channel means upon stacking and
that a stack of several layers each of said separator 1, electrode 2, and collector 3 is maintained in a condition compressed from both sides of the stack at a pressure of not less than 0.2 kg/cm2 G.
The method of treating a liquid according to the invention is characterized in that while a liquid is passed through a flow through capacitor satisfying all the conditions mentioned above, the liquid is treated by a suitable combination of (A) application of a DC constant voltage or DC constant current to collectors 3 with (B) short-circuiting between the collectors 3 or constant current discharge or of (A) application of said voltage and current, (B) short-circuiting or discharge, and (C) reverse connection between the collectors 3.